The invention relates to a method for starting an internal combustion engine having a fuel tank and a low pressure pump which conveys gasoline at a low pressure from the fuel tank to a high pressure pump which conveys the gasoline at a high pressure to injectors of the internal combustion engine, wherein an electronic control unit is provided for controlling and/or regulating the low pressure pump, the high pressure pump and the injectors.
At present, a constant low pressure is made available for the fuel supply of a gasoline internal combustion engine, which low pressure is subsequently increased substantially by way of a high pressure pump for the injectors. The low pressure is frequently adjusted via a mechanical pressure regulator which has an opening pressure of typically approximately 5.8 bar, relative (that is to say, with respect to ambient pressure). With the aid of the operating method described in German laid-open specification DE 10 2011 005 662 A1 for an electric fuel pump, the low pressure for the fuel supply of the gasoline internal combustion engine can be reduced as required, since the peak pressure of approximately 5.8 bar is necessary only in a few operating states.
Furthermore, German laid-open specification DE 10 2011 005 662 A1 discloses an operating method for an electric fuel pump which conveys fuel out of a tank of a motor vehicle to a consumer and, in particular, to the input side of a high pressure pump which provides said conveyed fuel at high pressure for the injection into the combustion chambers of an internal combustion engine. With regard to its conveying performance, the electric fuel pump is pilot controlled using a requirement characteristic diagram which takes an operating variable of the internal combustion engine and at least one further boundary condition into consideration.
The maximum possible pressure in the low pressure region of the fuel system also cannot be raised briefly in the case of a mechanical pressure regulator with an opening pressure of approximately 5.8 bar with respect to ambient pressure, in order to counteract the formation of vapor bubbles in the low pressure region of the fuel system. In the starting case of the internal combustion engine, if excessively high temperatures prevail at the low pressure components of the fuel system, this can lead to a starting failure. Formation of vapor bubbles is understood to mean the premature evaporation of highly volatile fuel constituent parts in the fuel system. As a result, the high pressure pump can no longer convey any fuel (compressible fuel vapor is present instead of incompressible fuel) and a hot start of the internal combustion engine is no longer possible in some circumstances.
It is an object of the present invention to provide a method for avoiding the formation of vapor bubbles at excessively high temperatures in the low pressure region of the fuel system.
This and other objects are achieved by a method for starting an internal combustion engine wherein after a starting pulse for the internal combustion engine, the method checks whether a formation of vapor bubbles of gasoline is possible on the low pressure side during a hot start, and if not, then the normal low pressure is set. If possible, then a low pressure is set which is sufficiently high that a formation of the vapor bubbles is not possible on the low pressure side. The normal low pressure is set after a defined time period.
It is therefore proposed according to the invention to raise the maximum possible pressure level in the low pressure region of the fuel system. This can take place, for example, by way of the installation of a mechanical pressure regulator with an opening pressure which is elevated to, for example, approximately 10 bar with respect to the surroundings (instead of the 5.8 bar pressure regulator). Suitable combinations of characteristic data from the electronics of the internal combustion engine are used to detect that a critical starting case, as described above, might be present (high temperatures in the low pressure region of the fuel system with the risk of a formation of vapor bubbles). In order to make a reliable hot start possible, the pressure on the low pressure side of the fuel system is briefly raised to such a level (for example, approximately 10 bar, relative) during starting of the internal combustion engine that a formation of vapor bubbles in the fuel is impossible.
Since the method according to the invention relates to only a very small proportion of all starts of the internal combustion engine, the measure has two advantages in contrast to a constant pressure supply with a constantly high operating pressure (for example, a constant pressure of 10 bar, relative):
1. The pressure loading of the components of the fuel system is greatly reduced. The components of the fuel system can be designed for 10 bar loading according to the rules of fatigue strength instead of for continuous 10 bar loading. This reduces the costs for all relevant components.
2. A brief increase in the operating pressure leads only as required to a somewhat increased energy consumption of the fuel pump which generates the pilot conveying pressure (low pressure part). In contrast, a constant increase in the pressure level in the low pressure region of the fuel system leads to an energy consumption which is considerably increased in comparison with today.
Advantageous developments of the method according to the invention are further described herein.
The check and decision as to whether a critical hot start is present takes place using various criteria. Thus, the check can take place over a time window after the internal combustion engine is switched off. The maximum temperatures during reheating occur only after a certain switched-off time.
Furthermore, the check can take place via the determination of the coolant temperature and/or the lubricant temperature of the internal combustion engine.
A further option for checking is via a control unit. A calculation model or a characteristic diagram can be stored in the control unit. The gasoline temperature can be calculated via the model or can be read out from the characteristic diagram.
Alternatively or additionally, it can be determined using the monitoring of the high pressure level (fuel pressure in the high pressure region, downstream of the high pressure fuel pump) whether the measured actual high pressure profile follows the setpoint high pressure profile. If the high pressure level collapses during or shortly after a hot start (actual pressure<setpoint pressure), a formation of vapor bubbles in the low pressure region of the fuel system can be assumed. In this case, the fuel low pressure is likewise raised briefly as needed, with the result that a further formation of vapor bubbles is avoided and the high pressure pump can again convey liquid fuel for the build-up of high pressure.
Lowering of the pressure level in the low pressure region of the fuel system to a lower operating pressure, for example 5.8 bar with respect to ambient pressure, can take place when the fuel temperature is at a lower level again and a formation of vapor bubbles is therefore no longer possible. Depending on the fuel consumption of the internal combustion engine, this can already take place very rapidly in some circumstances, since the fuel volumetric flow which is throughput generally cools the relevant hot part regions of the low pressure fuel system in a very satisfactory manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.